ADAU1590ASVZ AD [Analog Devices], ADAU1590ASVZ Datasheet - Page 15

• Manufacturer Part Number: ADAU1590ASVZ
• Description: Class-D Audio Power Amplifier
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
OVERVIEW
The ADAU1590 is a 2-channel high performance switching
audio power amplifier. Each of the two Σ-Δ modulators converts
a single-ended analog input into a 2-level PDM output. This
PDM pulse stream is output from the internal full differential
power stage. The ADAU1590 has built-in circuits to suppress the
turn-on and turn off pop and click. The ADAU1590 also offers
extensive thermal and overcurrent protection circuits.
MODULATOR
The modulator is a 5
stage connected internally. This helps reduce the external
connections. The 5
near full-scale inputs. The modulator gain is optimized at 19 dB
for 15 V operation. The Σ-Δ modulator outputs a pulse density
modulation (PDM) 1-bit stream, which does not produce
distinct sharp peaks and harmonics in the AM band like
conventional fixed-frequency PWM.
The Σ-Δ modulators require feedback to generate PDM stream
with respect to the input. The feedback for the modulators
comes from the power stage. This helps reduce the nonlinearity
in the power stages and achieve excellent THD + N perform-
ance. The feedback also helps in achieving good PSRR. In the
ADAU1590, the feedback from the power stage is internally
connected. This helps reduce the external connections for ease
in PCB layout.
The Σ-Δ modulators operate in a discrete time domain and
Nyquist frequency limit, which is half the sampling frequency.
The modulator uses the master clock of 12.288 MHz. This is
generated by dividing the external clock input by 2. This sets
the f
bandwidth of 22 kHz. The modulator shapes the quantization
noise and transfers it outside the audio band. The noise floor
rises sharply above 20 kHz. This ensures very good signal-to-
noise ratio (SNR) in the audio band of 20 kHz. The 6.144 MHz
bandwidth allows the modulator order to be set around the 5
order. The modulator uses proprietary dynamic hysteresis to
reduce the switching rate or frequency to around 700 kHz.
This reduces the switching losses and achieves good efficiency.
The dynamic hysteresis helps the modulator to continuously
track the change in PVDD and the input level to keep the
modulator stable.
SLICER
The ADAU1590 has a built-in slicer block following the PGA
and before the modulator. The slicer block is essentially a hard
limiter included for limiting the input signal to the modulator.
This, in turn, limits the output power at a given supply voltage.
The slicer in the ADAU1590 is normally inactive at lower input
levels but is activated as soon as the peak input voltage exceeds
the set threshold. The threshold can be set externally by
connecting a resistor from SLC_TH (Pin 24) to ground. This
S
/2 around 6.144 MHz. This is sufficient for the audio
th
-order modulator switches to a lower order
th
-order Σ-Δ with feedback from the power
Rev. 0 | Page 15 of 24
th
feature allows the user to adjust the slicer to the desired value
and to limit the output power. For input signals higher than the
set threshold, the slicer clips the input signal to the modulator.
This adds distortion due to clipping of the signal input to the
modulator. This is especially helpful in applications where the
output power available needs to be reduced instead of reducing
the supply voltage.
Figure 38 is a plot showing THD + N vs. the input level at
0 dB PGA, 12 V, and 6 Ω, and demonstrates the difference
between a device with and without the slicer.
Figure 39 depicts the typical output power vs. input at different
slicer settings.
From Figure 39, it can be seen that the slicer effectively reduces
the output power depending on its setting.
Internally, the slicer block receives the input from the PGA.
Figure 40 shows the block for slicer threshold adjust, SLC_TH
(Pin 24).
Figure 39. Typical Output Power vs. Input, at Different Slicer Settings
–100
12.0
11.5
11.0
10.5
10.0
–10
–15
–20
–25
–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
–5
0
0
Figure 38. THD + N vs. Input Level @ PGA = 0 dB, 12 V
0.1
0.1
0.2
0.2
0.3
0.3
0.4
0.4
0.5
INPUT (V rms)
0.5
INPUT (V rms)
SLICER DISABLED
0.6
0.6
SLICER DISABLED
SLICER 1.17V
SLICER 1.24V
SLICER 1.32V
SLICER 1.1V
0.7
0.7
SLICER 1.32V
SLICER 1.24V
SLICER 1.17V
SLICER 1.10V
0.8
0.8
0.9
0.9
ADAU1590
1.0
1.0
1.1
1.1
1.2
1.2